Optical communications systems typically employ semiconductor laser sources and glass optical fiber communication channels. There are many configurations of semiconductor lasers including various material compositions and various dimensions of the grown layers that form the active region and an associated optical waveguide in the laser structure. The material composition of the active region determines the wavelength of operation. For example, at lasing wavelengths around 9000 Angstroms, the III-IV materials of the ternary compound AlxGa(1-x)As with GaAs quantum wells provide a compact and rugged source of infrared light which can be easily modulated by varying the diode current. Light from a laser can be extracted by abutting an optical fiber thereto in known manner, however, devices fabricated in this manner do not lend themselves to semiconductor fabrication. Lasers can be abutted to optical fibers, however the indices of refraction between optical fibers and the semiconductor material forming the laser are so dissimilar that the amount of coupling may be very low, leading to an inefficient device. Furthermore, the alignment of the source with an optical fiber is quite tedious when high coupling efficiency is desired. This mismatch of the light field of the laser and that of the optical fiber also affect the amount of light coupled to the fiber.
For example, the output of a stripe heterojunction diode semiconductor laser, typically operated in the lowest order TE mode, has a highly asymmetric near field pattern (see end on view of FIG. 1a) and a wide vertical spread angle (numerical aperture .about.0.35, see elevation view of FIG. 1b) but a small horizontal spread (numerical aperture .about.0.1) due to the typically large difference in index of refraction between the active core and the cladding and the small height to width ratio of the cavity. In contrast, communication channel optical fibers which would carry the output of the semiconductor laser has a circular core (see end on view of FIG. 2a) and a small spread angle due to the small difference between the index of refraction of the core and the cladding (see elevation view of FIG. 2b); that is, the numerical aperture is symmetric and small (.about.0.15). Thus the direct coupling of the semiconductor laser output into an optical fiber has low efficiency due to mode field mismatching, and the usual approach to this problem inserts a lens between the laser and the optical fiber or to form the fiber end into a lens. FIG. 3a shows such a lens insertion to provide a high efficiency coupling, and FIG. 3b shows an optical fiber with its end rounded into a lens. Brenner et al, Integrated Optical Modeshape Adapters in InGaAsP/InP for Efficient Fiber-to-Waveguide Coupling, 5 IEEE Phot.Tech.Lett. 1053 (1993), describe a semiconductor waveguide with vertically tapered core and thickened cladding at a waveguide end for better coupling to a lensed optical fiber.
A previous co-assigned invention, (U.S. Pat. No. 5,673,284), the contents of which are incorporated herein by reference, introduced a semiconductor laser integrated with a silicon dioxide based waveguide having high efficiency coupling of the laser output into the waveguide by a integrated grating and a method of fabrication. This permits the laser output to be simply coupled into an optical fiber by a simple butt coupling of the optical fiber to the waveguide. Further, multiple lasers with different wavelengths can be integrated and their outputs coupled and combined into a single waveguide for wavelength division multiplexed operation.
In the device described in the above noted application, the direct coupling of the semiconductor laser output into an optical fiber was improved over the prior art by providing a semiconductor laser integrated with a silicon dioxide based waveguide which has far field mode shape nearly matches the optical fiber. The coupling from the laser to the co-integrated silicon oxide waveguide is achieved through the use of sub-mm gratings. The grating is integrated between the laser waveguide and the oxide waveguide, preferably in the laser waveguide. The grating, when appropriately designed such as the length, tooth height and period, provides a matching of the propagation in the laser with the propagation in the glass. The amount of light coupled from the laser waveguide to the glass waveguide ranges from 30 to 40 percent. The loss of power to the substrate is the major factor that limits the coupling efficiency.